Grease fittings--sometimes called "Zerk" fittings--have long been used in automobiles and a host of other apparatus for supplying grease to mechanical bushings and bearings without the need to disassemble the bushing or bearing to repack it and without the need to use bulky, awkward, and easily-damaged grease cups.
For years, grease fittings have been made from hex-shaped bar stock that is machined on an automatic screw machine. The bar stock is supplied in rather long lengths that have to be replenished manually at frequent intervals. Also a great deal of scrap is produced in the machining process, and the strength of the resulting fittings is limited by the machining process.
Grease fitting manufacture is largely automated and fairly inexpensive. However, a great many grease fittings are manufactured every year. Therefore, even a modest reduction in the cost of producing each grease fitting can translate into significant savings. Also, any improvement in the quality of the product can position a manufacturer more favorably with respect to its competitors.
Zerk fittings are usually sturdy and reliable. However, they are occasionally broken in service and must be extracted and replaced. Extraction is not usually difficult; because, when a grease fitting breaks in service, it is usually just below the nipple, leaving the threads and hex portion together. Also, if the fitting does break between the threads and the hex portion, there is a ready-made hole available for quick extraction of the remaining threaded portion. However, a stronger nipple structure might reduce the incidence of breakage in service.